Optical dispersive elements, such as diffraction gratings, prisms, and VIPA etalons, have been used in the art to separate spectrum of an input beam by dispersing its spectral components into different spatial directions.
The spatial separation of light into its spectral components facilitates various operation on the incoming radiation: (a) filtering the spectrum of said radiation by means of masks, apertures, etc., (b) tailoring the spectrum of said radiation by means of spatial light modulators, wave-plates etc., and (c) analyzing such spectrum by building light spectrometers in conjunction with a detector array or a CCD camera.
The quality of spectral manipulation or characterization can be measured by several parameters depending on the specific application and working conditions. The resolution of a device can indicate the minimal spectral separation that the device can detect or address; the throughput efficiency denotes the fraction of incoming light that is not lost; the sensitivity indicates the minimal power of light that is needed for the instrument to measure it or to work on it.
Among the various characteristics of a spectrometer, one that may be important for many applications is the dynamic range, or the ratio between the largest and the smallest measurable signal, in other words, the ability of the spectrometer to simultaneously measure signals of different strength. Ultimately, stray light in the spectrometer; extinction of the diffractive elements and dynamic range of the photodetecting device are the limiting factors for the spectrometer's dynamic range.
In a preferable device, when a monochromatic light passes through the diffractive element, it is likely redirected toward one direction which will correspond to the exact reading of its frequency (or wavelength); in practice, a small part of the incoming light always spreads in directions other than the ideal one. Extinction is the ratio between the intensity of the fraction of light emitted in a specific wrong direction and the intensity of the peak of light which is directed in the correct direction. Poor extinction may result in what is called, crosstalk, e.g., the unwanted leakage of a frequency component into a wrong measurement channel.
Traditionally, for spectral measurements that use high dynamic range and low crosstalk, monochromators have represented the only choice. In monochromators, one narrowband spectral component is measured at a time with a high dynamic range detector and a narrow slit minimizes instrumental stray light. Most importantly, monochromators can be easily cascaded to have multiple stages of spectral dispersion; at each stage, blocking masks and further spectral dispersion remove leakage due to imperfect gratings or stray light noise. Monochromators, though, are inherently slow because the measurement is performed sequentially. In addition, while their working principle is suitable for beam analysis, they cannot be used for the manipulation of the light spectrum.
For most applications in spectroscopy, it would be desirable to use spectrometers, where all the frequency components are measured in parallel and no moving parts are present. Unfortunately, parallel detection does not lend itself to multiple-stage extension for leakage reduction, prevents blocking stray light, and, employs array-detectors with limited dynamic range (typically, e.g., about 30-35 dB). As a result, spectrometers, although faster and more stable than monochromators, have not been competitive for high dynamic range applications.
Accordingly, there may be a need to overcome at least some of the deficiencies described herein above.